Railway cars typically consist of a rail car that rests upon a pair of truck assemblies. The truck assemblies include a pair of side frames and wheelsets connected together via a bolster and damping system. The car rests upon the center bowl of the bolster, which acts as a point of rotation for the truck system. The car body movements are reacted through the springs and friction wedge dampers, which connect the bolster and side frames. The side frames include pedestals that each define a jaw into which a wheel assembly of a wheel set is positioned using a roller bearing adapter.
The side frames and bolsters may be formed via various casting techniques. The most common technique for producing these components is through sand casting. Sand casting offers a low cost, high production method for forming complex hollow shapes such as side frames and bolsters. In a typical sand casting operation, (1) a mold is formed by packing sand around a pattern, which generally includes the gating system; (2) The pattern is removed from the mold; (3) cores are placed into the mold, which is closed; (4) the mold is filled with hot liquid metal through the gating; (5) the metal is allowed to cool in the mold; (6) the solidified metal referred to as raw casting is removed by breaking away the mold; (7) and the casting is finished and cleaned which may include the use of grinders, welders, heat treatment, and machining.
In a sand casting operation, the mold is created using sand as a base material, mixed with a binder to retain the shape. The mold is created in two halves—cope (top) and drag (bottom) which are separated along the parting line. The sand is packed around the pattern and retains the shape of the pattern after it is extracted from the mold. Draft angles of 3 degrees or more are machined into the pattern to ensure the pattern releases from the mold during extraction. In some sand casting operations, a flask is used to support the sand during the molding process through the pouring process. Cores are inserted into the mold and the cope is placed on the drag to close the mold.
When casting a complex or hollow part, cores are used to define the hollow interior, or complex sections that cannot otherwise be created with the pattern. These cores are typically created by molding sand and binder in a box shaped as the feature being created with the core. These core boxes are either manually packed, or created using a core blower. The cores are removed from the box, and placed into the mold. The cores are located in the mold using core prints to guide the placement, and prevent the core from shifting while the metal is poured. Additionally, chaplets may be used to support or restrain the movement of cores, and fuse into the base metal during solidification.
The mold typically contains the gating system which provides a path for the molten metal, and controls the flow of metal into the cavity. This gating consists of a sprue, which controls metal flow velocity, and connects to the runners. The runners are channels for metal to flow through the gates into the cavity. The gates control flow rates into the cavity, and prevent turbulence of the liquid.
After the metal has been poured into the mold, the casting cools and shrinks as it approaches a solid state. As the metal shrinks, additional liquid metal must continue to feed the areas that contract, or voids will be present in the final part. In areas of high contraction, risers are placed in the mold to provide a secondary reservoir to be filled during pouring. These risers are the last areas to solidify, and thereby allow the contents to remain in the liquid state longer than the cavity of the part being cast. As the contents of the cavity cool, the risers feed the areas of contraction, ensuring a solid final casting is produced. Risers that are open on the top of the cope mold can also act as vents for gases to escape during pouring and cooling.
In the various casting techniques, different sand binders are used to allow the sand to retain the pattern shape. These binders have a large affect on the final product, as they control the dimensional stability, surface finish, and casting detail achievable in each specific process. The two most typical sand casting methods include (1) green sand, consisting of silica sand, organic binders and water; and (2) chemical or resin binder material consisting of silica sand and fast curing chemical binding adhesives such as phenolic urethane. Traditionally, side frames and bolsters have been created using the green sand process, due to the lower cost associated with the molding materials. While this method has been effective at producing these components for many years, there are disadvantages to this process.
Side frames and bolsters produced via the green sand operation above have several problems. First, relatively large draft angles required in the patterns result in corresponding draft angles in the cast items. In areas where flat sections are required, such as the pedestal area on the side frames, and friction shoe pockets on the bolster, cores must be used to create these features. These cores have a tendency to shift and float during pouring. This movement can result in inconsistent final product dimensions, increased finishing time, or scrapping of the component if outside specified dimensions. Other problems with these casting operations will become apparent upon reading the description below.